In general, as a laminated plate, a laminated plate prepared by laminating a varnish of a thermosetting resin such as a phenol resin or an epoxy resin excellent in heat resistance, dimensional stability, strength, electric insulating property, etc., on a base material such as paper or a glass cloth has been widely utilized. However, such a laminated plate has problems in that the production equipment becomes large, the equipment cost becomes high, the productivity is low, and the cost of production is high.
Furthermore, a thermosetting resin is excellent in chemical resistance and water resistance, but when paper is used as the base material for the laminated plate, not only is the paper itself poor in chemical resistance but also the dimensional change by the moisture absorption is large, whereby the industrially usable range is considerably restricted. Also, there remains a problem in that in cutting the laminated plate, a cut powder forms, and further there is an inevitable fault in that three-dimensional molding is very difficult.
A thermoplastic polyester resin such as polyethylene terephthalate (hereinafter referred to as PET), which is excellent in its mechanical property, electric insulating property, heat resistance, chemical resistance, etc., is used in various industrial products and also is used as sheets.
In the case of producing a sheet from a thermoplastic polyester resin, a process of forming the sheet by melt-extruding the polyester resin from a T-die is employed. However, when a thick sheet is produced by the foregoing process, a temperature gradient occurs between the surface portions and the inside of the sheet, whereby a uniform sheet is not obtained.
As a process of producing a laminated plate of a thermoplastic resin, there is a process of obtaining the laminate by laminating plural sheets of the resin and hot-pressing the sheets. A vinyl chloride resin, etc., can produce a laminated plate by this process, but in the case of a thermoplastic polyester resin, when plural sheets of the resin are laminated and hot-pressed, the sheets do not join to each other, whereby a laminated plate can not be obtained.
Also, when a sheet is produced by melt-extruding a polyester resin filled with glass fibers from a T-die, the sheet has a smooth surface and a high elastic modulus in the lengthwise direction (hereinafter referred to as MD), but when the resin is melt-extruded from a T-die, the PET molecule and the glass fibers are oriented to provide the sheet, wherein the bending strength and the bending elastic modulus in the width direction (hereinafter referred to as TD) of the sheet are extremely lower than those in the MD.
JP-A-2-119011 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") proposes an extrusion-molded sheet composed of a polyester resin compounded with a granular inorganic compound for relaxing the orientation of a fibrous reinforcing material, and the effect thereof is large in regard to the anisotropy relaxation of the mechanical property. However, as described above, a uniform thick sheet cannot be obtained by extrusion molding only.